A shot sleeve is a device for injecting molten metal into a die or mold. Relatively simple in construction, it typically comprises a metal cylinder defining an axial chamber and a piston fitted within the chamber to act as an injection ram. An aperture in the side of the sleeve opens into a portion of the cylinder chamber just in front of the piston when it is in the rest position. This portion of the chamber is called the "well" and the molten metal is poured into the well for temporary residence before the piston is actuated.
Because of the high temperature difference between the molten casting metal and the elements of the shot sleeve, useful life expectancy of prior art devices is quite short. This is believed to be due in part to warpage and erosion of the axial chamber, and resulting piston wear.
Efforts have been made to increase useful life by water cooling the tip of the piston; i.e., the end whose face contacts the molten metal. It is theoretically possible to water cool the sleeve itself in the area of the well but both expense and the danger of ultimately leaking coolant into the well itself tend to discount the practicality of this approach.